a) Field of the Invention
Very high positioning accuracy is often required for switching optical components (objectives, reflectors, filters, etc.) arranged in changing magazines (nosepieces, slides, carriages or the like) in microscopes.
b) Description of the Related Art
Two principles are known for automatic, motorized switching:
1. Free positioning by means of a highly accurate drive, e.g., by a synchronous stepping motor with microstep control or by a drive that is regulated by a high-resolution position sensor. The individual switching positions are determined in advance so that they can be adjusted deliberately and quickly without further feedback (i.e., blindly) in an accelerating and braking process that is determined beforehand. In this connection, the strict requirements for the motor and gear unit, whose position resolution and reproducibility determine the positioning accuracy, are disadvantageous.
2. Positioning by means of a mechanical catch mechanism which, with the help of an energy accumulator (e.g., spring element), generates sufficient power within a determined capture area to move the changing magazine into its defined catch position in a precisely reproducible manner by overcoming the frictional forces. In this connection, the driving motor need only move the changing magazine out of its former catch position (against the retaining force of the catch mechanism) and guide it into the capture area of the target position.
For fast switching, the inert mass of the changing magazine must be sufficiently braked again when reaching the target position so that it comes to a stop as smoothly as possible in the catch and does not carry out an undesirable pendulum movement or even overshoot the target.
In the simplest case, the braking is caused by the mechanical friction of the gear unit after switching off the motor with no load. In an advantageous manner, the electromagnetic braking action of the driving motor is used additionally by electrically short-circuiting the driving motor. It is also possible to actively apply an opposite current pulse to the motor; but this heavily loads the motor and power supply and is difficult to apportion so as not to excite an opposite pendulum movement.
Previously known solutions provide transmitters which signal at the desired target position when the capture area is reached in order to turn off the drive and trigger the braking process. Additional transmitters can be used in order to distinguish between the individual catch positions in a definite manner by means of absolute coding so as to avoid stopping at and counting off the catch positions when initializing.
When the braking process is not triggered until the signaling of the transmitter in the capture area of the catch device, a high braking force is required because of the inertia of the changing magazine, subjecting the driving means to heavy electrical and mechanical stress which can cause troublesome vibration of the entire device. When the traveling speed is too great in proportion to the available braking force, the consumption of movement energy causes unwanted pendulum movements (rebounding or bouncing) in the capture area of the catch mechanism which retard the swinging-in process until the system is stabilized or can even cause the magazine to spring out of the capture area again. Therefore, the available braking force and the capture area define the permissible traveling speed and determine the total duration of the actual exchange and of the subsequent stabilization (swinging-in process). In many applications, a fast change process decisively influences the running time and cycle rate.
It is the primary object of the invention to maintain the lowest possible technical expenditure on the driving means, catch mechanism and sensors while realizing high traveling speeds.
The obvious solution, i.e., to expand the capture area of the catch mechanism and thus move up the signaling caused by it, is confined by technical limitations because the catch mechanism has only a limited energy accumulator which must develop adequate force over the entire length of the capture area to guide the changing magazine securely into its defined position against all friction forces, also without a motor drive (spring energy=integral of spring force x path over the capture area).
A broadening of the catch signal alone, so that the transmitter responds already before the capture area is reached, is problematic because an impermissible state can come about when the magazine remains stuck in an undefined intermediate position outside of the capture range but the transmitter signals a locked-in status. The catch signal should therefore be active only inside the capture area of the catch.
The invention meets the above-described object by combining the simple principle of a catch with a forward-looking operation having a determined traveling speed profile along path v(x) which can be determined beforehand corresponding to the catch positions xi which are assumed to be known. Accordingly, the braking process can be initiated already before reaching the capture area, that is, before the catch signal is initiated. In contrast to the free positioning with a stepping motor or position-regulated driving means, no particular precision is required because the capture area of the catch mechanism is capable of compensating for greater tolerances.